Christos Soulios d9f0245b10
[7.x] Implement stats aggregation for string terms (#49097)
Backport of #47468 to 7.x

This PR adds a new metric aggregation called string_stats that operates on string terms of a document and returns the following:

min_length: The length of the shortest term
max_length: The length of the longest term
avg_length: The average length of all terms
distribution: The probability distribution of all characters appearing in all terms
entropy: The total Shannon entropy value calculated for all terms

This aggregation has been implemented as an analytics plugin.
2019-11-15 14:36:21 +02:00

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[[search-aggregations-metrics]]
== Metrics Aggregations
The aggregations in this family compute metrics based on values extracted in one way or another from the documents that
are being aggregated. The values are typically extracted from the fields of the document (using the field data), but
can also be generated using scripts.
Numeric metrics aggregations are a special type of metrics aggregation which output numeric values. Some aggregations output
a single numeric metric (e.g. `avg`) and are called `single-value numeric metrics aggregation`, others generate multiple
metrics (e.g. `stats`) and are called `multi-value numeric metrics aggregation`. The distinction between single-value and
multi-value numeric metrics aggregations plays a role when these aggregations serve as direct sub-aggregations of some
bucket aggregations (some bucket aggregations enable you to sort the returned buckets based on the numeric metrics in each bucket).
include::metrics/avg-aggregation.asciidoc[]
include::metrics/weighted-avg-aggregation.asciidoc[]
include::metrics/cardinality-aggregation.asciidoc[]
include::metrics/extendedstats-aggregation.asciidoc[]
include::metrics/geobounds-aggregation.asciidoc[]
include::metrics/geocentroid-aggregation.asciidoc[]
include::metrics/max-aggregation.asciidoc[]
include::metrics/min-aggregation.asciidoc[]
include::metrics/percentile-aggregation.asciidoc[]
include::metrics/percentile-rank-aggregation.asciidoc[]
include::metrics/scripted-metric-aggregation.asciidoc[]
include::metrics/stats-aggregation.asciidoc[]
include::metrics/string-stats-aggregation.asciidoc[]
include::metrics/sum-aggregation.asciidoc[]
include::metrics/tophits-aggregation.asciidoc[]
include::metrics/valuecount-aggregation.asciidoc[]
include::metrics/median-absolute-deviation-aggregation.asciidoc[]